Experiments re Yield of Air-dry Bleached Pulp from Oran Esparto.
Air-dry Pulp containing 10 per cent. water.
| No. of Experiment. | Esparto. | Soda Liquor. | Conditions of Boiling. | Weight of Air-dry Pulp. Grams. | Dry Pulp on Dry Esparto. Per cent. | Bleaching Powder. Per cent. | |||
| Wt. taken. Grams. | Volume, C.C. | Per cent. Na2O. | Time. Hours. | Temp. °C. | Pressure. Lbs. | ||||
| 1 | 200 | 800 | 1·58 | 3 | 142 | 55 | 87·30 | 43·65 | 29·5 |
| 2 | 200 | 800 | 2·13 | 3 | 142 | 55 | 80·67 | 40·33 | 18·5 |
| 3 | 200 | 800 | 2·69 | 3 | 142 | 55 | 72·00 | 36·00 | 10·5 |
Practical Data calculated from Experiments.
| No. of Experiment. | Boiling. | Weight of Esparto to give 1 ton Pulp. Cwts. | 60 per cent. Caustic Soda required to Digest Esparto. Cwts. | Bleaching Powder required to Bleach 1 ton Air-dry Pulp. Cwts. | For One Ton of Esparto used. | ||
| Time. Hours. | Pressure. Lbs. | 60 per cent. Caustic. Lbs. | Bleaching Powder. Lbs. | ||||
| 1 | 3 | 55 | 45·8 | 4·30 | 5·26 | 210 | 260 |
| 2 | 3 | 55 | 49·5 | 6·27 | 3·39 | 282 | 156 |
| 3 | 3 | 55 | 55·5 | 8·90 | 1·96 | 358 | 79 |
Recovery of Spent Liquor.—As it is possible to recover 75 to 80 per cent. of the soda originally used in digesting the esparto, the washing of the boiled grass is conducted on scientific principles in order to ensure a maximum recovery of soda at a minimum cost.
The recovery is effected by evaporating down the black liquor, together with the washing waters, to a thick syrupy mass, which can be burnt. The organic and resinous constituents of the esparto which have been dissolved out by the caustic soda, forming the soluble soda compounds, ignite readily, and during combustion the organic soda compounds are converted more or less completely into crude carbonate of soda.
It is obvious, then, that the cost of recovery depends mainly on the quantity of weak washing water which has to be evaporated. Consequently methods are devised by means of which the grass is thoroughly washed with as little water as possible, and some of the methods are very ingenious.
The spent liquors and washing waters are evaporated to a small bulk in a vacuum multiple effect apparatus, and the thick liquid mass obtained by evaporation is burnt either in a rotary furnace or on an ordinary hearth. Every precaution is taken to effect this operation with a minimum quantity of coal. The burning off of this mass results in the formation of a black substance which is taken away from the furnace and allowed to char or slowly burn until the impure white soda ash, or carbonate of soda, is obtained.